This invention relates to an apparatus for generating a consistent and stable plasma. More particularly, it relates to an assembly (this assembly is also referred to as an “arc”) for generating a consistent and stable expanding thermal plasma (hereinafter referred to as “ETP”), which assembly is easy to maintain and operate.
Known methods for depositing an adherent coating onto a surface of a substrate by plasma deposition typically comprise passing a plasma gas through a direct current arc plasma generator to form a plasma. A substrate is positioned in an adjoining vacuum chamber (The vacuum chamber is also referred to as the “deposition chamber”). The plasma is expanded into the vacuum chamber towards the substrate. A reactant gas and an oxidant are injected downstream into the expanding plasma. Reactive species formed in the plasma from the oxidant and/or reactant gas contact the surface of the substrate for a period of time sufficient to form an adherent coating.
Plasma sources are used to provide a variety of surface treatments for a number of articles. Examples of such surface treatments include deposition of various coatings, plasma etching, and plasma activation of the surface. An array of multiple plasma sources may be used to coat or treat larger substrate areas. The characteristics of the plasma process are strongly affected by the operating parameters of these plasma sources.
Operating parameters typically used for the current arc design are the flow rate and pressure of the plasma gas, the electrical current applied to the arc and the voltage between cathode and anode. These operating parameters together with the arc geometry and design influence the degree of ionization of the plasma gas and hence surface properties and coating performance of parts coated in a plasma deposition process. In a typical plasma deposition process the gas flow rate and the arc current are controlled and result in control of the operating pressure and voltage.
During plasma treatment, conditions and geometry within the plasma source may drift, i.e. cathode voltage or operating pressure may change without changes in the current or gas flow. These changes can be attributed to a variety of causes within the plasma source. Sources of variability include changes brought about as a result of the erosion of the cathode. Other plasma source components subject to erosion include the cascade plate and the separator plate. During the operation of the plasma source copper can erode from the cascade plate and re-deposit across the insulator leading to reduced resistance between the two isolated plates and ultimately to shorting. Yet another cause leading to resistance changes or shorting of the arc is the presence of water between the electrically isolated plates, e.g. by a failure to exclude water from the environment or by leakage of coolant water into the interior of the plasma source. To counteract such drift, particularly the permanent changes caused by erosion of plasma source components, disruption of the plasma deposition process and disassembly of the plasma source are usually required.
An array of multiple plasma sources may at times be used to coat larger substrate areas. Ideally, the individual plasmas generated by each of the plasma sources in the array should have the same characteristics. In practice, however, source-to-source variation in plasma characteristics is frequently observed. Consequently, articles coated in a plasma deposition device comprising multiple plasma sources can demonstrate undesirable variability in surface coating properties at different locations on the coated substrate surface. Thus there is a need to reduce variability among multiple plasma sources in multi-source plasma deposition devices.
The plasma sources employed in plasma deposition devices have finite lifetimes and must be serviced or replaced periodically. Among typical plasma deposition devices, in order to service (i.e. repair or replace) the plasma source, the plasma deposition chamber must be vented to the atmosphere. Venting the plasma deposition chamber to the atmosphere requires that the plasma deposition process be shut down. This results in downtime and production losses. Furthermore the plasma source design typically comprises a variety of different components, which have to be machined to different tolerances. Thus, in some instances downtime for servicing the plasma source increases due to lack of availability of a component needed as a replacement part.
Typically, drift within a single plasma source cannot be corrected for in real time because such corrections require disruption of the process and disassembly of the plasma source. Where multiple plasma sources are used, minimization of source-to-source variation in the generated plasmas is often desirable. Therefore, what is needed is a simplified apparatus for the generation of a plasma, which apparatus is capable of generating a consistent and stable plasma, is easily serviceable, and which apparatus provides for greater efficiency in plasma mediated surface treatment processes, said efficiency being due in part to a reduction in apparatus downtime during servicing.